Einstein Rings: Nature s Gravitational Lenses

Transcription

1 National Aeronautics and Space Administration Einstein Rings: Nature s Gravitational Lenses Leonidas Moustakas and Adam Bolton Taken from: Hubble 2006 Science Year in Review The full contents of this book include more Hubble science articles, an overview of the telescope, and more. The complete volume and its component sections are available for download online at:

2

3 Einstein Rings: Nature s Gravitational Lenses Leonidas Moustakas and Adam Bolton In his General Theory of Relativity, published in 1915, Albert Einstein proposed that gravity bends the path of light. In 1936, at the urging of an amateur scientist, he wrote a brief paper about an optical illusion due to this bending: multiple images of one astronomical source located behind another. With near-perfect alignment, a full Einstein ring should appear around the intermediate, lensing object. Of course, there is no hope of observing this phenomenon directly, Einstein wrote. In his cover letter, he thanked the editor of Science for his cooperation with the little publication, which Mister Mandl squeezed out of me. It is of little value, but it makes the poor guy happy. Today, the elegant phenomenon of strong gravitational lensing the case when multiple images can occur makes many astronomers happy. Even though such gravitational lenses are uncommon, many have been found and studied, with Hubble playing an important role. Furthermore, lenses are proving to be much more than curiosities. When the lens and source are galaxies, the illusion can teach us about nonluminous and hence unseen dark matter, hidden structure, and the processes by which galaxies form and evolve. The angular size of the Einstein ring is determined by the amount of mass both stars and dark matter enclosed within it. By virtue of their stronger gravity, more massive foreground galaxies produce larger Einstein-ring images of galaxies in the background. Measuring the mass of galaxies is a difficult, but fundamental, task of astronomy. Lensing, when it occurs, is perhaps the most direct method. Though gravitational lensing has been studied previously by Hubble and ground-based telescopes, this phenomenon has never been seen before in such detail. The Advanced Camera for Surveys image of Abell 1689 reveals 10 times as many arcs as would be seen by a groundbased telescope. It is five times more sensitive and provides pictures that are twice as sharp as the previous Hubble cameras; it can see the very faintest arcs with greater clarity. 75

4 The Optical Illusion of Gravitational Lensing The more-distant galaxy is the source, and the intermediate galaxy is the lens. If the source, lens, and observer are closely aligned, the gravity of the lens will bend some light rays from the source onto new paths towards the observer. To the observer, these bent rays appear as if they originate at points on the sky displaced from the location of lens. Indeed, rays from multiple paths around the lens and at different clock angles may arrive at the observer, creating multiple images of the source. In the case of a near-perfect alignment, it is possible for light rays to be bent towards the observer all around the lens, creating the appearance of a full Einstein ring, as shown! An Einstein ring does not unambiguously tell how mass is arranged within the lensing galaxy. Nevertheless, the arrangement of mass is important for understanding the physical structure of the galaxy and its evolutionary history. We can solve this problem by measuring the distribution, or spread, of stellar velocities within the lensing galaxy: the faster the stars move, the more the mass must be concentrated towards the center of the galaxy, for gravity to balance centrifugal force. 76

5 Until recently, both diagnostic tools for studying galactic mass lensing and velocity spread were available only for a few objects. Out of the fewer than 100 gravitational lenses known, only a small number were sufficiently near, or luminous enough, to allow accurate measurements of their velocity spreads. To surmount this difficulty, astronomers are now combining two outstanding astronomical resources the Sloan Digital Sky Survey and the Hubble Space Telescope to find large numbers of new, bright, gravitational lenses, which they can study with great precision from both space- and ground-based observatories. Begun in 1998, the Sloan survey has imaged roughly one fourth of the sky and measured the brightness and colors of millions of stars, galaxies, and quasars. Using spectroscopy, Sloan has also measured the distances to nearly a million galaxies. (The distances are calculated from the redshifts of the galaxy spectra due to the cosmological expansion of the universe. See the sidebar on quasi-stellar object spectra in Arav s article on active galactic nuclei [AGN] outflow.) Any two objects that are found close together on the sky, but are located at vastly different distances, are prime candidates for gravitational lensing. By sifting through all the Sloan spectra of large, luminous galaxies, astronomers have discovered hundreds of new, bright candidates, which are usually massive elliptical galaxies in front of more distant, faint, star-forming galaxies. Observation Lens Lensed Source Flux Wavelength Sloan spectroscopy is key to finding gravitational lenses consisting of two galaxies lined up by coincidence. A difference in distance is indicated by a difference in cosmological redshift, that is, characteristic spectral features appearing at different, redshifted wavelengths. The left panel shows a typical spectrum that the Sloan spectroscopy might observe for a lens candidate (green). This candidate lensing galaxy is a giant elliptical galaxy, which exhibits the composite spectrum of myriad typical stars a spectrum that is well understood and can be easily modeled (middle panel, yellow). When the model spectrum is redshifted to match the absorption lines in the observed spectrum, and then subtracted from the observed spectrum, the spectrum of the distance source or lensed galaxy is revealed (blue). It shows anomalous emission lines at higher redshift than that of the lensing galaxy. 77

6 The Sloan-Hubble program has been extremely efficient in finding new Einstein rings. Two of these are shown here (top row). To make the study of the rings themselves easier, astronomers create simple models of the lens galaxies and subtract them out of the Hubble images. The results for these two lenses are shown in the bottom row. 78

7 Finding the true gravitational lenses among the Sloan candidates demands the unrivaled sensitivity and image sharpness of Hubble s Advanced Camera for Surveys (ACS). By the summer of 2006, images have confirmed more than 40 new gravitational lenses. Astronomers are studying the mass distribution in these lenses to better understand how galaxies form and evolve. Galaxy formation and evolution is a messy business, involving the gravitational force of both normal and dark matter, as well as the myriad processes of normal matter like star formation, radiation, cooling, turbulence, winds, and chemical enrichment. Adding to the complexity are collisions and mergers between galaxies, and the outflows and jets of supermassive black holes in AGN (which are the subjects of three other articles in this book). No models or simulations can yet take all these detailed processes into account. Nevertheless, a sufficient theoretical framework is available to make qualitative comparisons with the mass-related results of strong lensing. From Hubble images and Sloan spectra of strong lenses, astronomers gain three types of new information. First, the sizes of the Einstein rings provide the total masses of the lensing galaxies. Second, the images reveal the shapes of the lensing galaxies, which are useful for guiding the modeling of mass structure. Third, the Sloan spectra provide initial estimates of the velocity spreads. From this information, astronomers have reached a remarkable conclusion: The mass in the centers of elliptical galaxies has a simple, apparently universal structure, one that has remained the same over most of the age of the universe. Despite the varied, chaotic origins of these galaxies, their luminous and dark matter somehow interact to achieve a single end-state of mass structure. Furthermore, this end-state is very different from the structure predicted for the case of pure dark matter, which confirms the dominant role of the processes of normal matter in galaxy evolution. The Sloan-Hubble research program on gravitational lenses will discover more lenses and investigate this growing sample in greater detail. We expect to make a direct determination of how the ratio of total mass to luminosity in elliptical galaxies depends upon galaxy mass. The mass-to-light ratio is connected to the fraction of dark matter in the central region of a galaxy, and holds clues about how and when galaxies formed clues that we are still deciphering. A dependence of the ratio on galaxy mass has been seen in other types of observations, and it will be determined most accurately by gravitational lensing. 79

8 In the future, we expect that spectra from large ground-based telescopes will provide improved measurements of the velocity distributions in individual lensing galaxies, which will improve the accuracy of the measured mass distributions. This will lead to a better understanding of the relationships between the mass distribution and other properties of galaxies, such as the luminosity and the history of star formation. Sometimes an illusion is much more than it appears, as with Einstein s rings. Leonidas Moustakas is a Scientist at the Jet Propulsion Laboratory, which is operated by the California Institute of Technology. He has been involved in many searches for gravitational lenses from the Hubble Deep Field to the Hubble-Sloan program and is currently searching for gravitational lenses in all the Hubble data ever taken. He is especially excited about researching the evolution of the structure in the central regions of galaxies, which is only possible with gravitational lenses. Adam Bolton is currently a Postdoctoral Fellow at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. He devotes most of his time to the study of galaxy structure and evolution using strong gravitational lensing. The Hubble-Sloan project originally grew out of the research for his Ph.D. thesis at the Massachusetts Institute of Technology. His work in observational astrophysics combines dual interests in basic physics and amateur astronomy. The Hubble images seen here represent a significant improvement over the view through his 8-inch backyard telescope. The Sloan Lens ACS collaboration also includes Scott Burles, Leon Koopmans, and Tommaso Treu. 80

9 Two more examples of new lenses, as in the previous figure. The lens on the right is truly a cosmic bullseye, where the more-distant source galaxy is just about perfectly aligned with the massive lens galaxy and our view of it from Earth. It is a textbook case of an Einstein ring. 81

Galaxy Evolution is the study of how galaxies form and how they change over time. As was the case with we can not observe an individual galaxy evolve but we can observe different galaxies at various stages

Chapter 15.3 Galaxy Evolution Elliptical Galaxies Spiral Galaxies Irregular Galaxies Are there any connections between the three types of galaxies? How do galaxies form? How do galaxies evolve? P.S. You

What Are Stars? How are stars formed? Stars are formed from clouds of dust and gas, or nebulas, and go through different stages as they age. star: a large celestial body that is composed of gas and emits

Astronomy & Physics Resources for Middle & High School Teachers Gillian Wilson http://www.faculty.ucr.edu/~gillianw/k12 A cosmologist is.... an astronomer who studies the formation and evolution of the

Name Date Period 30 GALAXIES AND THE UNIVERSE SECTION 30.1 The Milky Way Galaxy In your textbook, read about discovering the Milky Way. (20 points) For each item in Column A, write the letter of the matching

Dark Matter and the Universe Topic 3 Dark Matter in Galaxy Clusters and Superclusters How does Einstein s bending of light reveal the true picture of our Universe?! Contents of Topic 3 In this Topic we

H9 Modeling the Expanding Universe Activity H9 Grade Level: 8 12 Source: This activity is produced by the Universe Forum at NASA s Office of Space Science, along with their Structure and Evolution of the

Origins of the Cosmos Summer 2016 Pre-course assessment In order to grant two graduate credits for the workshop, we do require you to spend some hours before arriving at Penn State. We encourage all of

OVERVIEW More than ever before, Physics in the Twenty First Century has become an example of international cooperation, particularly in the areas of astronomy and cosmology. Astronomers work in a number

GENERAL RELATIVITY & the UNIVERSE PCES 3.32 It was realised almost immediately after Einstein published his theory that it possessed solutions for the configuration of spacetime, in the presence of a homogeneous

What is the Sloan Digital Sky Survey? Simply put, the Sloan Digital Sky Survey is the most ambitious astronomical survey ever undertaken. The survey will map one-quarter of the entire sky in detail, determining

Cosmology - The Story of our Universe Raghu Rangarajan Physical Research Laboratory Ahmedabad What is Cosmology? Study of our Universe today galaxies, clusters, superclusters Understanding the past history

Notes on Ch. 23 and 24. The Milky Way Galaxy (ch. 23) [Exceptions: We won t discuss sec. 23.7 (Galactic Center) in class, but look it over in order to just get the most basic point I might put a question

Top 10 Discoveries by ESO Telescopes European Southern Observatory reaching new heights in astronomy Exploring the Universe from the Atacama Desert, in Chile since 1964 ESO is the most productive astronomical

The Milky Way Galaxy is Heading for a Major Cosmic Collision Roeland van der Marel (STScI) [based on work with a team of collaborators reported in the Astrophysical Journal July 2012] Hubble Science Briefing

THIRTY METER TELESCOPE The past century of astronomy research has yielded remarkable insights into the nature and origin of the Universe. This scientific advancement has been fueled by progressively larger

First Discoveries The Sloan Digital Sky Survey began operating on June 8, 1998. Since that time, SDSS scientists have been hard at work analyzing data and drawing conclusions. This page describes seven

Key Concepts: Lecture 33: The Big Bang! Age of the Universe and distances from H 0 Gravity slows down the expansion Closed, Flat and Open Universes Evidence for the Big Bang (Galactic Evolution; Microwave

Astro 130, Fall 2011, Homework, Chapter 17, Due Sep 29, 2011 Name: Date: 1. If stellar parallax can be measured to a precision of about 0.01 arcsec using telescopes on Earth to observe stars, to what distance

The Milky Way Galaxy Studying Its Structure Mass and Motion of the Galaxy Metal Abundance and Stellar Populations Spiral Structure and Star Formation The Milky Way Almost everything we see in the night

PLANCK'S VIEW OF THE UNIVERSE IN FRONT OF THE MICROWAVE BACKGROUND Gravitational lensing and Sunyaev-Zeldovich signals The Planck Consortium (presented by Simon White) Paris 21/03/2013 PLANCK'S FIRST IMAGE

Dark Matter and the Universe Topic 1 Dark Matter, Dark Energy and the Cosmology Revolution What is the real structure of our Universe and how do we know it is so? Contents of Topic 1 This first Topic provides

Suppose you tried to determine where we are in the galaxy by looking in different directions to see how many stars you could see. If you did this, you would pinpoint our location Suppose you tried to determine

A Universe of Galaxies Today s Lecture: Other Galaxies (Chapter 16, pages 366-397) Types of Galaxies Habitats of Galaxies Dark Matter Other Galaxies Originally called spiral nebulae because of their shape.

Study Guide Beginning Astronomy You must know these things: Earth's diameter is about 8000 miles Moon's distance is about 60 Earth radii (240,000 miles) Average distance of Earth to Sun is about 93 million

Chapter 5 Light and Matter: Reading Messages from the Cosmos Messages Interactions of Light and Matter The interactions determine everything we see, including what we observe in the Universe. What is light?

The Milky Way Galaxy Our Home Away From Home Lecture 23-1 Galaxies Group of stars are called galaxies Our star, the Sun, belongs to a system called The Milky Way Galaxy The Milky Way can be seen as a band

CHAPTER 2 4 Formation of the Universe SECTION Stars, Galaxies, and the Universe BEFORE YOU READ After you read this section, you should be able to answer these questions: What is the big bang theory? How

Big telescopes for small waves English summary of the inaugural lecture of dr. Floris van der Tak, on the occasion of his appointment as professor of submillimeter astronomy at the University of Groningen,

Specific Intensity Initial question: A number of active galactic nuclei display jets, that is, long, nearly linear, structures that can extend for hundreds of kiloparsecs. Many have two oppositely-directed

Using Photometric Data to Derive an HR Diagram for a Star Cluster In In this Activity, we will investigate: 1. How to use photometric data for an open cluster to derive an H-R Diagram for the stars and

Our Galaxy, the Milky Way In the night sky, the Milky Way appears as a faint band of light. Dusty gas clouds obscure our view because they absorb visible light. This is the interstellar medium that makes

Ch 6: Light and Telescope Wave and Wavelength..\..\aTeach\PhET\wave-on-a-string_en.jar Wavelength, Frequency and Speed Wave and Wavelength A wave is a disturbance that moves through a medium or through

The formation of the galaxy is believed to be similar to the formation of the solar system. All the gas & dust collapsed into a disk. During the time that stars were being formed, our galaxy didn t have

The Milky Way The Milky Way Almost everything we see in the night sky belongs to the Milky Way. 1 We see most of the Milky Way as a faint band of light across the sky. From outside, our Milky Way might

ASTRONOMY 113 EXAM #3: Covering Chapters 26-28 Time: 3:00-4:15pm VERSION C (KEY) Please write your GMU ID, Which test version you got (if not there will not be any way that I will be able to guess) Warning:

A scientific law is something that has been proven to be true. A. True B. False C. Only in experimental sciences. i>clicker Questions The fifth planet from the sun, the sixth planet and the seventh planet

Federation of Galaxy Explorers Space Science Once Upon A Big Bang Learning Objectives: 1. Explain how the universe was created using the Big Bang theory. 2. Understand how the existence of Cosmic Background

The Scale of the Universe Some Introductory Material and Pretty Pictures The facts we know today will be the same tomorrow but today s theories may tomorrow be obsolete. A scientific theory is regarded

The Birth and Assembly of Galaxies: the Relationship Between Science Capabilities and Telescope Aperture Betsy Barton Center for Cosmology University of California, Irvine Grateful acknowledgements to:

Spectroscopy, the Doppler Shift and Masses of Binary Stars http://apod.nasa.gov/apod/astropix.html Doppler Shift At each point the emitter is at the center of a circular wavefront extending out from its

Lecture Outline: Spectroscopy (Ch. 3.5 + 4) [Lectures 2/6 and 2/9] We will cover nearly all of the material in the textbook, but in a somewhat different order. First, we consider a property of wave motion,

AST1100 Lecture Notes 3 Extrasolar planets 1 Detecting extrasolar planets Most models of star formation tell us that the formation of planets is a common process. We expect most stars to have planets orbiting

Lesson 202: THE DOPPLER EFFECT The Doppler Effect is another type of illusion that led scientists to discover that the Universe is expanding. Fundamental Questions Attempting to give thorough and reasonable

Modern Ways of Dating the Universe Martha P. Haynes Goldwin Smith Professor of Astronomy Cornell University CAU Study Tour June 2014 What does the night sky look like? The disk of the Milky Way, our galaxy

Chapter 6 Telescopes: Portals of Discovery 6.1 Eyes and Cameras: Everyday Light Sensors Our goals for learning:! How does your eye form an image?! How do we record images? How does your eye form an image?

The Expanding Universe Prof Jim Dunlop University of Edinburgh Cosmology: The Study of Structure & Evolution of the Universe Small & Hot Big & Cold Observational Evidence for the Expansion of the Universe

Name: _Answer key Pretest: _2_/ 58 Posttest: _58_/ 58 Pretest Ch 20: Origins of the Universe Vocab/Matching: Match the definition on the left with the term on the right by placing the letter of the term

A short history of telescopes and astronomy: Galileo to the TMT Telescopes in the last 400 years Galileo 1608 Hans Lippershey applied for a patent for seeing things far away as if they were nearby 1609

Stars, Galaxies, Guided Reading and Study This section explains how astronomers think the universe and the solar system formed. Use Target Reading Skills As you read about the evidence that supports the

Elliptical galaxies: Ellipticals Old view (ellipticals are boring, simple systems)! Ellipticals contain no gas & dust! Ellipticals are composed of old stars! Ellipticals formed in a monolithic collapse,

SCIENCE 0 DISTANCES IN ASTRONOMY LECTURE NOTES Distances in the Solar System Distance to Venus can be obtained using radar ranging Send signal, determine how long it takes to return Radio waves move at

Astronomy Summer School in Mongolia National University of Mongolia, Ulaanbaatar July 21-26, 2008 Kaz Sekiguchi National Astronomical Observatory of Japan IYA2009 Galileo Cartoon Character Extra-Galactic

Earth Science Notes Packet #1 Unit 1: Astronomy, Part 1: The Big Bang 1.1: Big Bang The universe is How do we know this? All matter and energy in the universe was once condensed into a single point bya